1. Field of the Invention
This invention relates to a magnetic recording medium in which a magnetic wall is caused to displace to thereby reproduce information and a recording-reproducing method therefor.
2. Related Background Art
Attention is paid to magnetic recording media such as a magnetic recording medium and a magneto-optical recording medium for recording information thereon based on an orientation state of magnetization of a magnetic material and a recording-reproducing apparatus as a high density recording system capable of rewriting. In recent years, a demand has arisen for increasing the recording density of these magnetic recording media to thereby provide recording media of further large capacity.
In the magneto-optical recording system, the heat energy of a semiconductor laser is utilized to write a magnetic domain into magnetic thin film and record information, and the recorded information is read out by the use of the magneto-optical effect. Generally, the linear recording density of an optical recording medium depends greatly on the laser wavelength of a reproducing optical system and the numerical aperture NA of an objective lens. That is, when the laser wavelength .lambda. of the reproducing optical system and the numerical aperture NA of the objective lens are determined, the diameter of a beam waist is determined and therefore, the spatial frequency of a recording pit capable of reproducing a signal is limited to the order of 2NA/.lambda..
Accordingly, to realize higher density in a conventional optical disc, it is necessary to shorten the laser wavelength of the reproducing optical system or make the numerical aperture of the objective lens great. However, it is not easy due to the problems of the efficiency, heat generation, etc. of the element to shorten the laser wavelength, and if the numerical aperture of the objective lens is made great, the problem will arise that the depth of focus becomes shallow and the requirement for mechanical accuracy becomes severe.
Therefore, various so-called super-resolving techniques for contriving the construction of the recording medium and the reproducing method and improving the recording density without changing the laser wavelength and the numerical aperture of the objective lens have been developed.
For example, Japanese Laid-Open Patent Application No. 3-93058 proposes a signal reproducing method of effecting signal recording on a recording holding layer of multilayer film comprising a reproducing layer and a recording holding layer magnetically coupled together, uniformizing the direction of magnetization of the reproducing layer, thereafter irradiating the reproducing layer with a laser beam and heating it, and reading a signal recorded on the recording holding layer while transferring the signal to the temperature-increased area of the reproducing layer.
According to this method, relative to the spot diameter of a reproducing laser, an area which is heated by this laser and reaches a transfer temperature and in which a signal is detected can be limited to a smaller area and, therefore, the intersymbol interference during reproduction can be decreased and a signal of a period less than the diffraction limit of light becomes reproducible.
However, the conventional super-resolving system has adopted a method of masking part of the reproducing light, and limiting an aperture for substantially reading a pit to a small area to thereby increase the resolving capability. This has led to the problem that the light of the masked part becomes useless and the reproduction signal amplitude becomes small. That is, the light of the masked part does not contribute to the reproduction signal. Therefore, the more the aperture is narrowed in an attempt to increase the resolving power, the more the light effectively used decreases and the lower becomes the signal level.
In view of such a problem, Japanese Laid-Open Patent Application No. 6-290496 already proposes a method of displacing a magnetic wall present in the boundary portion of a recording mark by a temperature gradient by the use of a special magnetic recording medium, and detecting this displacement of the magnetic wall to thereby reproduce a high density recording signal.
This method, however, is a novel reproducing method entirely differing from the conventional reproducing system and therefore, the detailed conditions thereof have included many unknown portions.